Plastics can be recycled by a variety of methods, depending on the type of beginning polymer and the ultimate use of the recycled product. For example, post-consumer product polyester can be used in packaging applications, such as for beverages. Many of these recycling and recovery processes use various alternative processes to depolymerize the waste polyester to the monomer and then optionally filter and/or purify the monomer. The monomer can be then used as feedstock, for example, for polymerization with virgin raw materials. These processes all require additional steps or materials that lead to an increase in energy consumption.
The chemical treatment of poly(ethylene terephthalate) (PET), for instance, can be accomplished by hydrolysis, yielding terephthalic acid and ethylene glycol, or methanolysis, to provide di-methyl terephthalate (DMT) and ethylene glycol (EG), or glycolysis, which essentially reverses the polycondensation reaction to provide bis(hydroxyethylene) terephthalate (BHET) monomer.
Glycolysis of PET involves reacting waste PET with a glycol (such as EG) to produce the monomer bis-hydroxyethyl terephthalate (BHET) and other oligomers. This process has significant advantages over methanolysis or hydrolysis primarily because the BHET may be used as a raw material in either a DMT or terephthalic acid (TA) polyester production process without significant major modification of the production facility.
Generally, glycolysis reactions involve preparing the PET into the form of flakes, which are then soaked in a heated liquid having liquid EG at a concentration of twenty percent or more until the intrinsic viscosity is decreased to a desired extent. The liquid EG is used to break down the polymer but is not used for fine molecular weight control because it is difficult to precisely control. In yet another approach, liquid EG is injected into melted PET. The energy required to heat the liquid EG to the desired reaction temperature is significant, and the liquid EG must be recovered and purified, and then reheated to the desired reaction temperature.
Currently, processing of post-consumer PET includes a decontamination step whereby recycled PET flakes are melted under a vacuum to remove volatile contaminants and filtered to remove particulates. The filtered molten PET is then pelletized, crystallized, and solid state polymerized until a desired molecular weight range is achieved. The pellets are then reheated, dried, and reextruded into bottles, sheets, or films. The reheating and reextruding steps require additional energy and capital costs.